Bile pigments are linear tetrapyrrolic compounds which are important in nature. They occur in both animals and plants, having quite different functions in each. In animals, particularly mammalian systems, they result from the catabolism of hemoglobin and other heme proteins. Improper elimination of bile pigments, particularly bilirubin, can lead to severe pathological conditions such as jaundice. In plants they act as photoreceptors. Bile pigments and related linear tetrapyrroles are also important in synthetic chemistry as intermediates in porphyrin synthesis. Much chemical work has been done on these compounds in an attempt to elucidate their structures and conformation. Many controversies exist as to the conformation of the bile pigments and the effect it has had on the physiological role of these compounds. Non-bonded interactions, particularly hydrogen bonding, are believed to have a profound effect. The most powerful tool of structure determination, x-ray crystallography, has only recently been used to any great extent in the study of the bile pigments. It is the purpose of this proposal to continue such an investigation. A number of bile pigments and their derivatives have been obtained from various sources, and it is hoped to carry out determinations of their structures, and thus to answer many of the questions about this class of compounds.